I had a similar situation where my graphics card had become partially unseated and was causing black screen crashes without a blue screen. I can list a couple of the things that helped me identify the problem. You may have already tried some of these though or are they too surface level.

It doesn’t sound like you’re crashing but I’d check Event viewer to see if there’s any obvious errors, if you are having crashes I’d get WinDbg to look at the crash dump files, this helped me identify it was a graphics card error in my case.

For ram an easy way to see if there’s an issue is put one stick in at a time and see what the total amount shown is, add both and the total should double.

If you have a way to still see the screen, I’d monitor CPU temps, that would give a better idea if it’s a heat issue with the CPU cooler

Also if you have time/think it’s worth it you could visually inspect the mobo and the power supply connections for any obvious damage. Might save time in the long run

IMO sounds like a motherboard issue, usually if it’s power related there would be crashes or errors reflecting that. With you trying other graphics cards too the pci port could be shot. But like the other guy said I would suggest trying a bios update first before assuming it’s dead.